GaN (gallium nitride) is one of group III-V compound semiconductors and has a wurtzite crystal structure belonging to hexagonal crystal systems.
In recent years, single crystal GaN wafers have attracted attention as semiconductor wafers for nitride semiconductor devices.
Nitride semiconductors are also referred to as, for example, nitride-based group III-V compound semiconductors, group III nitride compound semiconductors, and GaN-based semiconductors, and include, in addition to GaN, a compound in which a part or all of gallium in GaN is substituted with another group 13 element (such as B, Al, and In).
One of highly useful single crystal GaN wafers is a C-plane GaN wafer. The C-plane GaN wafer is a single crystal GaN wafer having a main surface parallel to or slightly tilted from the C-plane. The C-plane GaN wafer has a gallium polar surface which is a main surface on the [0001] side and a nitrogen polar surface which is a main surface on the [000-1] side. So far, it is mainly the gallium polar surface that is used to form nitride semiconductor devices.
Preferable methods for growing GaN single crystals used for the C-plane GaN wafer include an ammonothermal method. In the ammonothermal method, GaN dissolved in ammonia in a supercritical or subcritical state is precipitated as a single crystal on a seed.
Non-Patent Document 1 reported that a C-plane GaN wafer was produced from a GaN single crystal grown by an ammonothermal method, and that a dislocation-free area of 1 mm2 was observed on the surface of the C-plane GaN wafer.
Non-Patent Document 2 reported that a C-plane GaN wafer of 2 inches in diameter was fabricated from a GaN single crystal grown by an ammonothermal method.
Patent Document 1 described that a pattern mask having linear openings was formed on a main surface of a C-plane GaN wafer used as a seed and a GaN layer was grown through the openings by an ammonothermal method. The extending direction of the linear openings was the m-axis direction <10-10> or the a-axis direction <11-20>. According to the description, GaN crystals grown from the inside of the linear openings of the pattern mask were further grown in a lateral direction on the pattern mask, and coalesced with each other to form a single layer.
Patent Document 2 described that a pattern mask having linear openings was formed on a nitrogen polar surface of a C-plane GaN wafer used as a seed and GaN single crystals were grown through the linear openings by an ammonothermal method. According to the description, the GaN crystal grown through each of the linear openings was further grown by as long as 10 mm in the [000-1] direction without coalescence.
Non-Patent Document 3 reported growth rates of GaN crystals obtained when various ammonium halide mineralizers were used in an ammonothermal method.